Conventional pipettes generally include a cylindrical body, a cylindrical piston within a cavity of the cylindrical body, and a mechanical or automated actuating mechanism for actuating the cylindrical piston. When the actuating mechanism causes the cylindrical piston to perform an upward stroke, liquid is aspirated into a pipette tip attached to the end of the cylindrical body. When the actuating mechanism causes the cylindrical piston to perform a downward stroke, liquid is dispensed from the pipette tip. The diameter of the cavity and the diameter of the piston define a volume range for the pipette. The volume range can refer to the range of volumes which the pipette is able to reliably aspirate and dispense. In general, large diameters correspond to a large volume range and small diameters correspond to a small volume range. For example, a cylindrical pipette in which the cavity and the piston have a relatively small diameter may have a volume range of 1-10 μL. A cylindrical pipette in which the cavity and the piston have a larger diameter may have a volume range of 100-1000 μL. Because of their uniform nature, conventional cylindrical pipettes can only aspirate and dispense within a single volume range.
A multivolume pipette is a pipette which is capable of reliably operating over more than one volume range. U.S. Pat. No. 4,679,446 describes a multivolume pipette in which the cavity of the pipette body is composed of sections, each of which has a distinct diameter. The multivolume pipette also includes a piston which is composed of sections with distinct diameters. A seal surrounds each piston section to form a plurality of chambers within the cavity. Each chamber includes a channel opening which is in fluid communication with an outside environment when no pipette tip is installed on the pipette body. As such, this multivolume pipette requires pipette tips of varying sizes to control the volume range of the pipette.
For example, a first pipette tip may cover only the bottommost channel opening such that the bottommost channel opening is no longer in communication with the outside environment. As such, the bottommost chamber is able to build pressure and is an operational pipette chamber. The other chambers are not able to build pressure because they are still in fluid communication with the outside environment. Thus, the multivolume pipette can operate within a first volume range corresponding to the bottommost chamber. A second (larger) pipette tip may cover the bottommost channel opening and the next successive channel opening such that the first two chambers become operational chambers. As such, the second pipette tip causes the pipette to operate in a second volume range, where the second volume range is larger than the first volume range. A third pipette tip can be used to provide a third volume range, and so on. In addition to requiring specially tailored pipette tips, this multivolume pipette is also limited because it does not provide any mechanism to ensure that a user selects the proper pipette tip.
U.S. Pat. No. 3,640,434 describes another multivolume pipette in which the piston and the cavity are each composed of sections of increasing diameter. The chambers formed by the piston and the cavity have channel openings which are in fluid communication with an annular space within the pipette body. A user of the pipette can rotate a ring to cause an elliptical seal to move inside the annular space such that the chambers can be placed into or out of communication with one another to control the volume range of the pipette. For example, positioning the elliptical seal such that only the bottommost chamber is in communication with the annular space corresponds to a first volume range. Positioning the elliptical seal such that the two bottommost chambers are in communication with the annular space corresponds to a second (larger) volume range, and so on. This multivolume pipette is limited in part because of the high risk of user error involved in manually placing the elliptical seal. Further, the elliptical seal is subject to wear and tear as it is repeatedly moved along the cavity. A deformed or otherwise damaged elliptical seal can impair the ability to isolate cavities from one another and lead to inaccurate volume transfers. In addition, this multivolume pipette requires at least two operations by the user to perform a volume transfer. The user must adjust the elliptical seal to obtain the proper volume range and also set the piston stroke to obtain the correct volume within that volume range.
Thus, there is a need for a multivolume pipette which is capable of operating in all volume ranges with a single pipette tip. There is also a need for a multivolume pipette in which the risk of operator error is minimized. Further, there is a need for a multivolume pipette in which a specific volume in a specific volume range can be selected by a single user action.